c function not returning result

Question

I've very recently started to learn C, so I realize my question is very basic, but any help would be very much appreciated.

I'm trying to get the function fact to return the res value to main, but when I print out the result in main I just get 0. By inserting some print statements I can see that res is calculating correctly in the fact routine but the result is not returning correctly to main.

I'm sure I'm missing something very basic here.

Thanks

#include <stdio.h>

unsigned long fact (int n){
    unsigned long res = 1;

    while ( n >= 0 )
    {
        res *= n;
        n--;
    }

    return res;
}

int main (void){
    int n;
    unsigned long res;

    printf("Insert number:\n");
    scanf("%d", &n );

    res = fact (n);

    printf("The factorial number is %lu", res);

    return 0;
}

Answer 1

Your loop condition is n >= 0, which means that res will be multipled by 0 before the function returns. Thus the result will always be 0.

Answer 2

You loop condition is wrong. The last run of while (n>=0) will have n=0. Multiplying res by this will reset it to 0.

You can fix this by changing your loop to while (n > 1)

For future reference, you could investigate problems like this using a debugger (e.g. GDB or visual studio express). Or by adding printf statements to your code to trace the flow and see how the value of res changed through the program.